The invention relates to a light grid for the detection of objects in a monitoring region having a transmitter unit comprising a plurality of light transmitters and a receiver unit comprising a plurality of light receivers in which respective pairs of light transmitters and light receivers associated with one another and bounding the monitoring region can be activated in succession in time in dependence on a synchronization signal transmitted between the transmitter unit and the receiver unit. The invention further relates to a method for the operation of such a light grid.
Light grids of this kind are used, for example, in highly automated production systems in which work pieces are automatically fed, machined and taken away again. These production systems are provided as a rule with fully automatic machining centers (e.g. robot devices) and transport devices. Since the feeding, machining and taking away regions at the same time represent possible danger regions, for example for operators, it is necessary with such systems to provide protection against non-permitted intrusion. Such protection can be achieved by means of light grids of the kind initially mentioned.
Since synchronous operation between the light transmitter and the light receiver is required for the operation of a light grid, it is usual in light grids in accordance with the state of the art to emit a synchronization signal coupled to a first monitoring signal, for example from the first light transmitter of the transmitter unit, in the direction of the first light receiver of the receiver unit in order to activate it. Subsequent to this synchronization procedure, all further pairs of light transmitters and light receivers associated with one another are then activated in succession according to a given timetable in order to thus realize the desired monitoring function. The disadvantage of this procedure is that whenever the synchronization signal is interrupted, for example by a work piece moving into the monitoring region, no synchronization can take place between the transmitter unit and the receiver unit and thus the operation of the light grid is interrupted. It is therefore customary practice to deactivate light grids for, for example, the time of the moving in and out of an object, whereby disadvantageously no protection against non-permitted intrusion exists in this time.